1. Technical Field
This invention relates generally to magnetically levitated high-speed spindle assemblies for shaping non-circular surfaces, and more particularly toward such an assembly for forming a non-circular hole in a workpiece.
2. Related Art
Some production applications require the formation of non-circular holes. For example, in the manufacture of pistons for an internal combustion engine, the so-called pin holes formed for the wrist, or gudgeon pin often have a slight trumpet shape opening toward the center for accommodating flex in the wrist pin. Additionally, this trumpet shape of the pin hole is optimally designed with a slight ovality to further account for distortion in the wrist pin as it moves through its various cycles. This trumpet shape and non-circular cross section of the pin hole must be formed to exacting tolerances. For example, tolerances in the range of 3 to 5 microns is often required for these types of applications.
There are industrial methods for creating such non-circular holes to exacting tolerance specifications, including hydraulically actuated milling tools and electro chemical machining techniques. The prior art has also suggested boring non-circular shapes using a special machine tool spindle equipped with active magnetic bearings. Active magnetic bearings act upon the principle that the rotating spindle is formed of or with a ferromagnetic material that is supported in a magnetic field generated by an electromagnet stator. A control system, with appropriate power amplifiers, modulates the magnetic field to maintain the spindle in the desired radial position while it rotates. This radial position can be maintained even under changing load conditions.
The advantage of an active magnetic bearing system resides in its ability to control spindle movement and intentionally introduce deviations in the spindle rotations to thereby move the shaping tool in a desired path. Numerous shortcomings and limitations in the prior art nevertheless exist. For example, the non-circular hole shapes which can be produced, particularly in 3 dimensions, is especially limited. And further, the ability to precisely control the position of the tool cutting edge and thereby yield tolerance variations less than 5 microns is not reliable. It is, therefore, an object of the invention to overcome these disadvantages and limitations.